In polyurethane foam manufacturing, surfactants are needed to stabilize the foam until the product-forming chemical reactions are sufficiently complete so that the foam supports itself and does not suffer objectionable collapse. On account of the complex interplay of these physico-chemical and rheological phenomena, it is not readily possible to predict the effect of subtle compositional changes on the overall performance of a surfactant even for those skilled in the art.
High potency silicone surfactants, generally understood to be those which give a high height of rise and little top collapse at minimal use levels, are desirable because foams which collapse to a substantial degree before setting have high densities and objectionable density gradients. In general, it is preferred that the surfactant produces high rise, little or no top collapse, and high airflow performance. The latter feature refers to the ability of air to pass through the foam and is also referred to as foam breathability.
Silicone surfactants for polyurethane foam manufacture typically are materials having siloxane backbones and polyether pendant groups. For example, U.S. Pat. No. 4,147,847 describes certain polysiloxane-polyoxyalkylene copolymer ("Copolymer") surfactants having mixed alkylene oxide feed polyethers with molecular weights up to approximately 5500. U.S. Pat. No. 4,025,456 teaches that a key to excellent performance is using a blend of polyethers with a distribution of molecular weights where a significant part of high molecular weight polyether is preferred.
With regard to such teachings, unfortunately, conventional alkylene oxide polymerization catalysts such as KOH cannot produce high quality polyethers with molecular weights above 5000 if more than about 20% propylene oxide (PO) is present in the alkylene oxide feed. Since the prior art teaches the need for the use of PO (or higher alkylene oxides) in the polyethers, this is a serious limitation. With conventional catalysts such as KOH, small amounts of PO continuously rearrange to give allyl alcohol which functions as a new source of unsaturated starter in competition with the original starter. Eventually conditions are established where further PO addition fails to increase the overall molecular weight of the polyether product. In other words, in attempting to increase the molecular weight, more low molecular weight species are generated which compete with the existing oligomers for chain growth and the overall number average molecular weight of the polyether product does not increase. With KOH catalysis, for example, the overall number average molecular weight levels off around 5000 daltons for these mixed polyethers.
Moreover, due to the reactivity of KOH, the polyethers produced thereby do not have a random distribution of alkylene oxide units when a mixed feed is used. Instead, when a polyether is prepared from a blended feed of ethylene oxide (EO) and propylene oxide (PO), the distal portions of the polyether (from the starter) are rich in PO as compared to the proximal end thereof. Said lack of even distribution affects polyether performance.
When analyzed by size exclusion chromatography, high molecular weight, i.e., &gt;5000 MWt, polyethers made with substantial amounts of PO and KOH catalysis exhibit a broad distribution of molecular weights (generally having a polydispersity of greater than 1.4) and contain a substantial amount of low molecular weight polyether contaminant. These low molecular weight contaminants compete with the high molecular weight polyethers during synthesis of the silicone surfactants and effectively reduce the number of high molecular weight pendants bound to the silicone backbone. Only the lower MWt polyethers (i.e., &lt;5000 MWt) typically have a polydispersity &lt;1.5. Since the art teaches that high molecular weight pendants are important for potency, a substantial content of lower molecular weight polyethers are not contributing to good performance and therefore are undesirable.
Double metal cyanide (DMC) catalysts have been used in silicone surfactant preparation as reported in Japanese Kokai 05-117,352 which discloses the DMC synthesis of allylpoly(PO) polyethers of conventional molecular weights and subsequent addition of ethylene oxide (EO) moieties to these products using conventional KOH technology (final molecular weights less than 3000 daltons). Accordingly, the polyethers disclosed therein have a blocked, non-random distribution of EO and PO units. Moreover, this process necessitates the extra step of removing the KOH prior to the subsequent hydrosilation reaction because residual KOH reacts preferentially with hydridosiloxane (SiH) functional groups and reduces the efficiency of polyether grafting to the siloxane backbone during hydrosilation and produces hydrogen gas, a process hazard.
European Patent Application 0,573,864 A2 discloses the use of DMC catalysts for the addition of epoxides (PO and allyl glycidyl ether) to non-hydrolyzable siloxane-polyether copolymers having uncapped hydroxyl groups. Thus, the epoxides were added directly to the polysiloxane, rather than forming a polyether separately as is standard in the art. Such a synthesis provides no ability to achieve a surfactant where the pendant polyethers have varied molecular weights or compositions.